Configurable path for iconv.h and pthread.h

i noticed this in Debian mailing list. It would be great to help Debian maintainers so that they won't have to maintain this patch.

Link to the original discussion: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=911574

Source: zabbix
Version: 1:4.0.0+dfsg-1
Tags: patch upstream
Control: block 798955 by -1

The m4 files used for generating zabbix' configure script hard code the
locations of iconv.h and pthread.h. This will stop working with
non-glibc libcs as well as a glibc that fully moves headers to
/usr/include/<triplet> (#798955). zabbix will fail to build as is.
Please consider applying the attached patch.